Open Estuaries Research Articles

The marine fish assemblage of the East Kleinemonde Estuary over 20 years: Declining abundance and nursery function?

Marine fishes in the intermittently open East Kleinemonde Estuary, South Africa, were sampled using seine nets over a twenty year period between 1994 and 2014. This dataset was analysed after 11 years (1994–2005), and represented one of the few medium-term studies of fish communities in an intermittently open estuary. A further nine years of sampling (2006–2014) allowed us to revisit the fish community and to gauge if 20 years of data provides more insight into long-term trends in the fish community. The overall species abundance follows a cyclical pattern with two to three years of low abundance and two to three years of high abundance. This pattern broke down in the last five years of the study, with a decline in species richness and consistently low catches of almost all marine species recorded. This may have been as a result of a reduction in the area of submerged macrophytes in the estuary, as well as an increase in the frequency of mouth opening and subsequent decrease in available water area.

Similar habitats, different communities: Fish and large invertebrate assemblages in eastern Gulf of Mexico polyhaline seagrasses relate more to estuary morphology than latitude

Seagrass habitats are a dominant component of coastal waters along the eastern Gulf of Mexico coast and are recognized as essential habitats for many species. Although various ecologically and economically important species depend on seagrass habitats at some life stages, these habitats are vulnerable to anthropogenic influences. As coastal human populations continue to grow, and nearshore habitats are affected, understanding the structure and function of assemblages associated with nearshore habitats is important for management and mitigation efforts. Therefore, we sampled estuarine and nearshore polyhaline seagrass beds monthly (May–November) from 2008 through 2015 using a 6.1-m otter trawl in seven estuaries in the eastern Gulf of Mexico. Despite latitudinal variability, assemblage structure of fishes and selected larger invertebrates was predominantly driven by estuary morphology—semi-enclosed estuaries had significantly higher catch-per-unit-effort (CPUE) of estuarine obligates and incidental marine taxa, whereas open estuaries had higher CPUE of small forage and cryptic species. Furthermore, abundances of several important fishery species differed markedly between semi-enclosed and open systems. Our results highlight (1) the relative importance of different scales of environmental factors' influence on communities, (2) the need for understanding how seemingly similar habitats in estuaries of differing morphologies can support different fishery species, and (3) the importance of regional-scale monitoring data and its value in tracking ecological changes.

Illustrating the value of cross-site comparisons: Habitat use by a large, euryhaline fish differs along a latitudinal gradient

Plasticity in life-history traits and behaviors allows organisms to track spatial environmental variation, such as that resulting from a latitudinal gradient. In estuaries, hydrological patterns can vary greatly with latitude, causing variable habitat use patterns across a species’ range. For Common Snook (Centropomus undecimalis) a large euryhaline fish, seasonal movements between the open estuary and freshwater reaches of rivers have been documented. We hypothesized that the timing of river use by Snook would be driven primarily by the magnitude of freshwater inflow, except at the northern limit of its range where lethal water temperatures may be a stronger influence. Data from electrofishing studies (2012–2016) conducted in rivers associated with four estuaries along a latitudinal gradient in western Florida were compared for seasonal differences in Snook abundance using negative binomial regression. In rivers of southwestern Florida (those in Everglades and Charlotte Harbor), Snook abundances increased three-fold during the time of year when surface waters inundating floodplains recede and force prey into the main stems of rivers. These hydrologically driven processes, and surges in Snook abundance, occurred during spring in the Everglades and fall in rivers of Charlotte Harbor. In Tampa Bay rivers, which have less freshwater inflow than the rivers farther south, Snook abundance differed little between seasons. In spring-fed rivers north of Tampa Bay, Snook abundances in fall and winter were twice those of spring and summer; stable water temperatures were thought to provide thermal refuge at the northernmost range of Snook. This work illustrates how the timing of habitat use by a species, and the underlying reasons, can differ greatly among sites. It also demonstrates the value of cross-site comparisons in providing for a more complete understanding of how a species uses available habitats across its range.

Centennial record of anthropogenic impacts in Galveston Bay: Evidence from trace metals (Hg, Pb, Ni, Zn) and lignin oxidation products

During the 20th century the impacts of industrialization and urbanization in Galveston Bay resulted in significant shifts in trace metals (Hg, Pb, Ni, Zn) and vascular plant biomarkers (lignin phenols) recorded within the surface sediments and sediment cores profile. A total of 22 sediment cores were collected in Galveston Bay in order to reconstruct the historical input of Hg, Pb, Ni, Zn and terrestrial organic matter. Total Hg (T-Hg) concentration ranged between 6 and 162 ng g−1 in surface sediments, and showed decreasing concentrations southward from the Houston Ship Channel (HSC) toward the open estuary. Core profiles of T-Hg and trace metals (Ni, Zn) showed substantial inputs starting in 1905, with peak concentrations between 1960 and 1970's, and decreasing thereafter with exception to Pb, which peaked around 1930–1940s. Stable carbon isotopes and lignin phenols showed an increasing input of terrestrial organic matter driven by urban development within the watershed in the early 1940s. Both the enrichment factor and the geoaccumulation index (Igeo) for T-Hg as a measure of the effectiveness of environmental management practices showed substantial improvements since the 1970s. The natural recovery rate in Galveston Bay since the peak input of T-Hg was non-linear and displayed a slow recovery during the twenty-first century.

Acoustically Measured Distribution and Abundance of Atlantic Menhaden (Brevoortia tyrannus) in a Shallow Estuary in Long Island, NY

Atlantic menhaden (Brevoortia tyrannus) is an euryhaline forage fish abundant along the eastern coast of the USA that seasonally migrates between the open ocean and shallow water estuaries. These fish are known for forming large, dense schools and are important prey for piscivorous fish, seabirds. Following several large fish kills in the late spring of 2015 in the western Peconic Estuary in Long Island, New York, we conducted multiple surveys during the spring, summer, and fall using scientific echosounders to quantify menhaden abundance in this region. Acoustic backscatter data were used to estimate numeric abundance and biomass of menhaden for the western Peconic Estuary. We found that the 2015 fish kills represented between 7 and 18% of the total local population (~ 2.4 million) that were present at the time. Significant changes in target strength distributions during the study suggest that the size classes of menhaden present in the estuary change as fish grow or migrate in and out of the estuary. This study demonstrates that vertical beam active acoustics is a viable method for quantifying forage fish in a very shallow (< 4 m depth) estuary.

Ichthyofaunal assemblages in shallow littoral habitats of permanently open estuaries and intermittently closed and open lakes or lagoons in Otago, New Zealand

Fish assemblages of New Zealand estuaries are poorly studied, and knowledge of the effects of estuary–ocean connections on the ichthyofaunal composition of estuaries remains limited. Understanding the status of fish composition of estuaries is crucial for planning for sustainable management of aquatic ecosystems. In the present study we sampled fish using a seine net from lower reaches of six permanently open estuaries and six intermittently closed and open lakes or lagoons (ICOLLs) along the Otago coastline during winter 2016 and summer 2017. Marked differences in ichthyofaunal composition were observed in the shallow littoral habitats of permanently open estuaries and ICOLLs. Fish assemblages reflected estuary–ocean connection status of estuaries during both seasons. ICOLLs showed greater fish abundance than permanently open systems. Fish abundance was higher in summer than in winter in both estuary types. Fish species with marine–estuarine opportunist and estuarine–migrant life histories dominated permanently open estuaries. Conversely, species with a diadromous life history but known to form landlocked populations were abundant in ICOLLs. Salinity and temperature were correlated with fish abundance in both estuary types.

Observed and predicted impacts of climate change on the estuaries of south-western Australia, a Mediterranean climate region

Regions with a Mediterranean climate are generally predicted to become warmer and drier with climate change. Estuaries in these regions are influenced by a broad range of climate drivers and are particularly vulnerable to the effects of climate change. We examine observed and predicted effects of climate change on the estuaries of south-western Australia (SWA), where sustained warming and drying trends have caused dramatic declines in freshwater flows of up to 70% since the 1970s, as a case study of the impacts that might be expected in other Mediterranean regions. Current and projected impacts of climate change in SWA include progressive warming and ‘marinisation’ of estuaries; extended closure of periodically open systems; an increased frequency and severity of hypersaline conditions; enhanced water column stratification and hypoxia; and reduced flushing and greater retention of nutrients. We document the effects of these environmental changes on the habitats, biota and ecology of SWA estuaries, including phytoplankton, macrophytes, invertebrates and fish. For example, decreasing river flows will cause periodically open estuaries across SWA to remain closed for longer periods, inhibiting the extent to which marine taxa can access these systems, thus reducing species diversity, whereas marinisation of permanently open systems will increase species diversity. We discuss the broader relevance of our findings, placing them in a global context and highlighting implications for ecosystem services and human populations. Finally, we consider the adaptation options that could be implemented to reduce the impacts of climate change in Mediterranean climate regions.

Temperature-induced variability in metabolic activity of ecologically important estuarine macrobenthos

Temperature is one of the more important factors affecting metabolism and fitness of aquatic ectothermic animals. The present study aimed, therefore, at quantifying and comparing the oxygen consumption rates of several subtropical macrobenthos organisms living in two South African estuarine systems. Oxygen fluxes were measured in the laboratory at an individual level for temperatures ranging between 16 and 30 °C. The metabolic response differed significantly among taxa, developmental stage, and microhabitat, with higher activation energy for the animals from the permanently open estuary compared to the one from the temporarily open/closed system. The invasive snails showed an exceptional higher metabolic performance at higher temperatures compared to the other organisms collected within the temporarily open/closed system. The results suggest that estuaries experiencing prolonged closure with stable environmental conditions and lower biodiversity host the least performing native species. Setting this pattern in a future global change scenario, this study highlights that longer closures of the estuarine mouth caused by changing rainfall regimes and increased freshwater abstractions from catchments are likely to create the ideal conditions for the proliferation of more adaptable invasive species in temporally open/closed subtropical estuaries.

Eelgrass beds and bare substrata – sparid and mugilid composition in contrasting littoral estuarine habitats

Seagrass beds serve as nursery grounds for many fish species and often play an important role in the juvenile stages of economically and recreationally important fishes. The eelgrass Zostera capensis is the dominant submerged macrophyte in permanently open South African estuaries and occupies large intertidal and subtidal areas within the Knysna system. The primary objective of this study was to compare the occurrence of sparids and mugilids in eelgrass and nearby bare sediment areas using sampling with seine nets. The hypothesis that was tested is that mugilids are dominant in unvegetated areas of the Knysna Estuary littoral whereas sparids predominate within eelgrass beds located in the same zone. The results indicate that the family Mugilidae is better represented at unvegetated sites when compared to members of the family Sparidae, with the exception of Lithognathus lithognathus, but that the dominant three sparids and dominant two mugilids were most abundant in sparse eelgrass beds that included both bare and vegetated areas within this habitat type. The main fish species responsible for the separation of fish assemblages associated with unvegetated sites versus those associated with vegetated areas were Rhabdosargus holubi, Liza dumerili, Liza richardsonii, Lithognathus lithognathus, Sarpa salpa and Diplodus capensis, with R. holubi, S. salpa and D. capensis being mainly attracted to eelgrass habitats and L. dumerili, L. richardsonii and L. lithognathus having a stronger affinity for bare areas. This finding confirms a previous estuarine study in the nearby Swartvlei lake littoral, which found that sparids dominated areas where aquatic macrophytes were extensive and that mugilids became more prevalent at those same sites when the macrophytes underwent complete senescence. The length-frequency distributions of some sparid and mugilid species differed considerably between eelgrass and bare sediment areas, whereas those of other species showed little or no difference between these two habitat types.

Modelling the occurrence of postflexion stages of a marine estuarine-dependent fish in temperate South African estuaries

The movement of postflexion larvae of marine estuarine-dependent species into estuaries is critical for the survival of fishes reliant on estuaries as nurseries. However, detailed studies focused on environmental variability experienced by postflexion larvae entering a range of estuary types under varying conditions are rare. This study assessed the in situ conditions (temperature, salinity and water clarity) under which the southern African endemic fish Rhabdosargus holubi (Sparidae) recruits into estuaries. Postflexion larvae were sampled in three biogeographic regions (cool temperate, warm temperate and subtropical boundary), which included three estuary types (permanently open estuaries (POEs), temporarily open/closed estuaries and estuarine lake systems) on a seasonal basis, independent of each other. Rhabdosargus holubi larvae were more abundant in spring and summer, in POEs in the warm temperate region. Models predicted that higher larval occurrence in estuaries is a function of lower salinity (e.g. mesohaline zones of 5-17.9 salinity) and lower water clarity (e.g. 0-0.2 Kd, light extinction coefficient), particularly for warm, temperate POEs. This re-emphasizes the importance of freshwater for optimal nursery functioning, which may be compromised by impoundments, abstraction and climate change in water-short countries like South Africa.

Short food chains, high connectance and a high rate of cannibalism in food web networks of small intermittent estuaries

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 587:17-30 (2018) - DOI: https://doi.org/10.3354/meps12401 Short food chains, high connectance and a high rate of cannibalism in food web networks of small intermittent estuaries Vanessa Mendonça, Catarina Vinagre* MARE - Marine and Environmental Sciences Centre, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal *Corresponding author: cmvinagre@fc.ul.pt ABSTRACT: Small intermittent estuaries are a common feature in some parts of the world. They are recognized as important refuge and nursery areas for several marine species. However, their biological dynamics are still poorly known. In fact, the network structure of their food webs remains undescribed. Highly resolved food webs were compiled for 23 small intermittent estuaries in the Northeast Atlantic, Mediterranean, Southwest Indian Ocean, and South Pacific. The structural network properties of these food webs were compared with those of larger open estuaries, lakes, and rivers, as well as marine and terrestrial ecosystems. The most important conclusion is that the network properties of these systems are different from those of larger open estuaries and more like other non-estuarine ecosystems. They also stand out from all other ecosystems, including larger open estuaries, in terms of omnivory, which was found to be remarkably high (83-92%), probably due to the prevalence of opportunistic feeding. Nevertheless, all other properties were well within the ranges reported for other ecosystems; thus, the general organization rules of food web networks also apply to small intermittent estuaries. It was concluded that general rules applied to the management of estuaries should be looked at carefully when managing these smaller intermittent systems, since they have shorter chain lengths, higher connectance, and more cannibalism. Shorter chain length implies that disturbance is more likely to rapidly affect many food web components through predator-prey links, which may be counterbalanced by the effect of high connectance. These systems should, thus, not be looked at simply as smaller versions of larger open systems. KEY WORDS: Estuarine ecosystems · Coastal lagoons · Niche model · Network theory Full text in pdf format Supplementary material PreviousNextCite this article as: Mendonça V, Vinagre C (2018) Short food chains, high connectance and a high rate of cannibalism in food web networks of small intermittent estuaries. Mar Ecol Prog Ser 587:17-30. https://doi.org/10.3354/meps12401 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 587. Online publication date: January 25, 2018 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2018 Inter-Research.

Benthic trophic status of aquatic transitional environments with distinct morphological and dynamic characteristics on the south-western Atlantic coast

The benthic trophic status of Uruguayan coastal estuarine habitats (permanently open estuaries and open or closed coastal lagoons) was evaluated, twice in 1 year and at different spatial scales, using the amount and biochemical composition of the sedimentary organic matter. Nested hierarchical ANOVAs were applied to evaluate differences at the habitat level, among sites for each type of estuarine habitat (three estuaries and three lagoons) and between sectors located at different distances from the sea (inner v. outer sectors). Morphological and hydrodynamic differences between the two types of habitats explained site-to-site variations in eutrophic conditions in the open or closed coastal lagoons and meso-oligotrophic conditions in permanently open estuaries. These differences followed the patterns found for sediment grain size, with inner sectors (lower-energy areas) favouring the accumulation of finer sediments and higher amounts of degraded–detrital organic matter. In autumn, biochemical parameters indicated the dominance of aged and more degraded organic matter, with both types of habitats having a low nutritional value no matter which sector was considered. By contrast, in spring, biochemical composition showed the prevalence of fresh and more labile sedimentary organic matter with a high food value, especially in the outer sectors.

Are nest sites a limiting resource for speleophilic nesting fishes in sand dominated temporarily open/closed estuaries?

Southern African temporarily open/closed estuaries are often characterized by sand dominated mouth regions, a feature in contrast to many permanently open estuaries of the region which regularly have substantial rocky areas. Here we explore how habitat characteristics may reduce nesting opportunities and potentially explain population trends for a common estuarine fish species, the speckled sand goby Psammogobius knysnaensis, in many temporarily open/closed estuaries in South Africa. The present study is the first to document that P. knysnaensis is a speleophilic nester, utilizing the underside of rocks and other hard substrata for egg attachment. Furthermore, our results suggest that ideal nesting habitat for the speckled sand goby is potentially a limiting resource in sand-dominated temporarily open/closed estuaries. We postulate that this is a factor contributing to the relatively low numbers of adult P. knysnaensis found in sandy temporarily open/closed estuaries of the region, although the role of marine connectivity, food availability, predation pressure and other environmental factors in this regard still remain largely unassessed.

Testing the efficacy of an estuarine eutrophic condition index: Does it account for shifts in flow conditions?

Eutrophication of estuaries via anthropogenic nutrient enrichment is an issue being addressed extensively on a global scale, however it remains a topic that garners further attention due to its complexity. The aim of this study was to test the efficacy of a recently proposed eutrophic condition index in five permanently open estuaries, subsequent to flow variations. Further, in order to elucidate their potential role in augmenting eutrophic conditions, the influence of vegetated habitats on associated microalgal communities was investigated. The study took place in the summer of 2014 and 2015. On each sampling occasion, all of the proposed ‘state’ indicators (nutrients, oxygen and microalgal communities) were assessed and supplemented with a heuristic ‘pressure’ component (flow conditions). Overall, the proposed index was shown to be sensitive to environmental perturbations providing shifts in classification ratings illustrating the dilution (Swartkops Estuary) or augmenting (Kromme, Gamtoos, and Kariega estuaries) effect of freshwater pulse events. Additionally, the sensitivity of selected epiphyte and microphytobenthos (MPB) parameters to water quality variations was verified, thus supporting their inclusion as indicators in the proposed index. Regarding the role of microhabitats in promoting microalgal growth, it was found that MPB biomass was higher (5–290%) and benthic diatom diversity generally lower (5–50%) in vegetated compared to unvegetated habitats – an important consideration when applying the index. The habitat complexity and stabilisation provided by estuarine macrophytes (Phragmites australis and Zostera capensis) supported notable autotroph colonisation. This highlights their importance in structuring trophic pathways, whilst also potentially providing a corridor for the intensification of eutrophic symptoms in estuaries. Overall, this study provides an important step towards the verification of a proposed assessment methodology which may serve to provide a baseline from which the eutrophic status of estuaries can be monitored – particularly in countries where such assessment frameworks are lacking.

Sequential development of tidal ravinement surfaces in macro- to hypertidal estuaries with high volcaniclastic input: the Miocene Puerto Madryn Formation (Patagonia, Argentina)

The late Miocene beds of the Puerto Madryn Formation (Provincia del Chubut, Argentina) are formed by shallow marine and estuarine sediments. The latter include several tidal-channel infills well exposed on the cliffy coast of the Peninsula Valdes. The Bahia Punta Fosil and Cerro Olazabal paleochannels are end members of these tidal channels and show a fining-upward infilling starting with intraformational channel lag conglomerates above deeply erosional surfaces interpreted as fluvial ravinement surfaces (the erosion surface formed in the purely fluvial or the fluvially dominated part of the estuary, where erosion is driven by fluvial processes). These are overlain and eventually truncated (and suppressed) by the tidal ravinement surface (TRS), in turn covered with high-energy, bioclastic conglomerates mostly formed in the “tidally dominated/fluvially influenced” part of an estuary. Above, large straight or arcuate point bars with alternatively sandy/muddy seasonal beds and varying trace and body fossil contents were deposited from the freshwater fluvially dominated to saline-water tidally dominated part of the estuary. The upper channel infill is formed by cross-bedded sands with mud drapes and seaward-directed paleocurrents, together with barren, volcaniclastic sandy to muddy heterolithic seasonal rhythmites, both deposited in the fluvially dominated part of the estuary. Volcanic ash driven by the rivers after large explosive volcanic eruptions on land resulted in sedimentation rates as high as 0.9 m per year, preserving (through burial) the morphology of tidal channels and TRSs. The channel deposits were formed in a tide-dominated, macrotidal to hypertidal open estuary with well-developed TRSs resulting from strong tidal currents deeply scouring into the transgressive filling of the channels and eventually cutting the fluvial ravinement surface. The TRSs extended upstream to the inner part of the estuary during long periods of low sedimentation rates, extended channel migration and sediment bypass, interrupted by transient, high volcaniclastic input. The tidal channels of the Puerto Madryn Formation constitute a unique example of estuary sedimentation with pulsed sediment supply in a macrotidal to hypertidal estuary.

A comparison of CO2 dynamics and air‐water fluxes in a river‐dominated estuary and a mangrove‐dominated marine estuary

AbstractThe fugacity of CO2 (fCO2 (water)) and air‐water CO2 flux were compared between a river‐dominated anthropogenically disturbed open estuary, the Hugli, and a comparatively pristine mangrove‐dominated semiclosed marine estuary, the Matla, on the east coast of India. Annual mean salinity of the Hugli Estuary (≈7.1) was much less compared to the Matla Estuary (≈20.0). All the stations of the Hugli Estuary were highly supersaturated with CO2 (annual mean ~ 2200 µatm), whereas the Matla was marginally oversaturated (annual mean ~ 530 µatm). During the postmonsoon season, the outer station of the Matla Estuary was under saturated with respect to CO2 and acted as a sink. The annual mean CO2 emission from the Hugli Estuary (32.4 mol C m−2 yr−1) was 14 times higher than the Matla Estuary (2.3 mol C m−2 yr−1). CO2 efflux rate from the Hugli Estuary has increased drastically in the last decade, which is attributed to increased runoff from the river‐dominated estuary.

Geological Assessment of the Khor Kharfot Sediments, Western Dhofar Region, Sultanate of Oman

Khor Kharfot is part of a coastal estuarine wetland system located in the western part of the Dhofar Governorate preserving a pristine ecosystem supporting a distinct vegetation and animal habitat. A sand barrier along the khor (local term for wetlands) mouth controls the mixing of fresh and marine water in the enclosed lagoon. In order to understand the geological controls on the khor formation, a geological map of the khor barrier and inlet area is prepared. The khor is bounded by Cretaceous and Tertiary carbonate rock sequences which are cut by a number of faults. The khor mouth and barrier are comprised of late Holocene deposits divided into seven units including i) beach sand, ii) beach sand mixed with wadi gravels, iii) scree and coastal dunes, iv) lagoon, v) soil and boulders, vi) wadi gravels, and vii) talus. The khor developed as an open estuary which changed to a barrier dominated lagoon due to falling fresh water supplies as a consequence of dwindling Khareef rains. The lagoon water is brackish due to the mixing of fresh and marine water and supports a distinct flora and fauna.

Recovery of nitrogen stable isotope signatures in the food web of an intermittently open estuary following removal of wastewater loads

Nitrogen (N) stable isotope values (δ15N) were used to assess the removal of wastewater N from the food web within Tallow Creek, a small intermittently closed/open lake/lagoon (ICOLL) on the east coast of Australia, following the cessation of wastewater inputs in 2005. Current (2013) δ15N values of sediment organic carbon, plants, and animals within Tallow Creek were compared to values obtained before wastewater inputs ceased, and to values within a nearby near-pristine ICOLL (Jerusalem Creek). Most biota had significantly depleted δ15N values compared to conspecifics collected before wastewater inputs ceased (mean reduction of 6.0‰; 38% of impacted enrichment), indicating substantial loss of wastewater N since inputs ceased. However, δ15N values remained enriched compared to the near-pristine ICOLL for some components (mean enrichment of 3.3‰ or 38%), suggesting that some wastewater N remains. The δ15N recovery rate (decrease in δ15N as a percentage of the impacted enrichment) for Tallow Creek biota was slow compared to that of biota in more open systems. This slow recovery rate and the persistence of some wastewater N, even after 8 years without new inputs, reflects differences in hydrology and nitrogen cycling between permanently open and intermittently open estuarine systems and highlights the likely lower resilience of ICOLLs to anthropogenic N inputs.

Distribution of macrophyte species and habitats in South African estuaries

This study investigated the area coverage and species composition of different habitat types in all estuaries and bioclimatic regions along the coast of South Africa. Estuary habitat covers a total area of 95,657ha. Reeds and sedges (14,732.6ha) were the dominant habitat type overall. Supratidal salt marsh was dominant in the cool temperate region (4869.7ha), intertidal salt marsh in the warm temperate region (2509ha) and reeds and sedges in the subtropical region (10,484ha). In the cool temperate, warm temperate and subtropical regions, the estuaries with the largest areas were the Berg (6799ha), Knysna (2038.7ha) and St Lucia estuaries (40,832.8ha), respectively. The number of macrophyte species per estuary varied from one to 53. Macrophyte species richness was higher in estuaries of the cool and warm temperate zone than that in the subtropical zone. Macrophyte species are distributed in 47 families and the dominant families are the Cyperaceae (18), Chenopodiaceae (14), Juncaceae (5), while 15 other families had only one representative species. The number of species in temporarily open/closed estuaries (TOCEs) and permanently open estuaries (POEs) was higher than that found in other estuary types (river mouths, estuarine bays and lakes). Only Juncus kraussii and Phragmites australis occurred in more than half of South African estuaries. The data are collated in an estuary botanical database that can be accessed for monitoring and conservation planning purposes. These findings have already found application in the management of estuaries in South Africa, the National Biodiversity Assessment and the National Estuary Biodiversity Plan.

Estuarine characteristics, water quality and heavy metal contamination as determinants of fish species composition in intermittently open estuaries

Estuaries are critical aquatic environments that are used by many fish during their life cycle. However, estuaries often suffer from poor water quality as a result of anthropogenic activities. Fish diversity studies in estuaries are common, although few have examined whether correlations exist between water quality, metal contamination and fish assemblages. In the present study we investigated the effect of abiotic conditions, heavy metals and estuary characteristics on the abundance, diversity and composition of fish in four intermittently open estuaries along the Illawarra coast of south-eastern Australia. The heterogeneity of environmental conditions was reflected in the fish assemblages in each estuary. Environmental variables predicted fish species composition, and estuaries in particularly poor condition contained few species (estuarine residents) in high abundance, indicating their ability to acclimatise and survive in conditions that are hostile to other species. Overall, these findings demonstrate that estuarine fish assemblages may be useful indicators of estuary condition and reveal the importance of managing anthropogenic activities in the surrounding catchment to improve water quality so that biodiversity of fish can be restored in these estuarine environments.